In plasma cutting technology, making use of a torch comprising a refractory electrode and a tube for producing of a plasma, an electric arc is struck between the electrode and the workpiece which is to be cut. An ionised gas flowing within the interstitial space between the electrode and tube and finally through a tube constriction establishes the conditions for producing a plasma arc of appropriate characteristics.
The electrical power is then supplied by an electric d.c. generator the negative terminal of which is connected to the torch electrode and the positive terminal is connected to the workpiece.
In the application of this plasma cutting technique, it is appropriate to make provision for feeding the torch towards the workpiece which is to be cut up. The torch is fed to a correct working distance, and the cutting arc is established by closing the power supply circuit of the electric generator, commonly however after striking an auxiliary arc of low power between the torch electrode and tube, in such manner as to ionize the plasmagenic gas. In one form of application intended more particularly for automatisation, the plant comprises a mechanism for transferring the torch into position, for example a toothed rack and a forward and reverse motor, the whole being installed on a carriage for transverse displacement of the torch depending on the joint which is to be cut; means are incorporated for detecting the correct working position, which are of the capacitive probe type operating with a fixed frequency oscillator (for example 20 kHz) having voltage as a direct function of the gap between the workpiece and the probe which is, itself, integral with the torch. By means of an electronic circuit, this voltage is compared to a datum value which is picked by the operator of the torch to provide the correct spacing for the cutting operation.
This solution implies the presence of a detection device on the torch, and of an electronic processing circuit completing the control circuits of the torch-workpiece displacement mechanism, which represents a complex plant. In addition, the selection of the torch-workpiece spacing depends on the individual judgement of the operator. The choice of this spacing between the torch and the workpiece is of great importance since a slightly excessive gap does not allow the arc to be established and a slightly inadequate gap may cause damage to the torch tube by molten metals spattering, caused by violent striking of the arc.
It is an object of the invention to provide a process and a plant for automatic ignition of a plasma cutting torch which are uncomplicated in application and are particularly convenient in construction.